mmc: sdhci: add support for realtek rts5250

Add support for realtek rts5250 pci card reader. The card reader has
some problems with DDR50 mode, so add a new quirks2 for broken ddr50.

Signed-off-by: Micky Ching <micky_ching@realsil.com.cn>
Signed-off-by: Chris Ball <chris@printf.net>